Originally, highway utility poles were wood, steel or concrete, but such poles provided rigid resistance to impact from vehicles. As a result, when a vehicle struck such a utility pole the occupants of the vehicle were commonly subjected to excessive deceleration forces that often result in extremely severe, or even fatal, injury. This result led to the development of highway utility pole structures which greatly reduced not only the deceleration forces but also the high incidence of sever, or fatal, injuries. In fact, utility poles utilized on federally funded highway projects must now meet rigid breakaway performance criteria. The presently required breakaway performance criteria are set forth in the American Association of State Highway Transportation Officials Publication entitled "Standard Specifications for Structural Supports for Highway Signs, Luminaries and Traffic Signals 1985."
To meet these requirements, highway utility poles have been constructed of lightweight materials such as aluminum and fiber reinforced plastic (FRP). FRP materials--typically reinforced with fiberglass--have proven to be particularly desirable not only in meeting the breakaway performance criteria, but also because such utility poles are not electrically conductive. Thus, when FRP poles are broken the pole itself can not serve as a conductor. This is a definite advantage attributable to the FRP utility pole.
Electrical service is frequently provided to utility poles by underground lines, and the hollow interior of even the non-conductive utility poles serves as the path along which the electrical service lines may extend to provide the desired electrical power to lights, or the like, mounted at the top of the pole, or to whatever height required. An access port is typically provided in the utility pole, normally at a height which can be reached by a worker standing on the ground or on a relatively short ladder. Such access ports are each provided with a closure hatch assembly that employs an adjustable mounting means, such as a screw and bracket, or bar, to secure the closure hatch, or plate, in a position over, or within, the access port. The bracket engages the interior surface of the hollow pole to provide a base into which the screw can be tightened in order demountably to secure the closure hatch in position.
The presence of an access port, however, has been found to reduce the bending strength of the utility pole at that location where the port penetrates the wall of the utility pole, and that location is normally above that at which the pole is intended to fail when struck by a vehicle. The reduced bending strength of the utility pole results not only by virtue of the absence of wall material at the location of the port but also because most access ports are configured such that stress concentrations are induced, particularly at any locations around the perimeter of the access port which presents a relatively sharp corner. A significant reduction of stress concentrations can be accomplished by eliminating any vestige, of sharp corners, and by providing rounded surfaces to define the perimeter of the access port. Even so, the very existence of the access port has heretofore resulted in an unavoidable reduction in the bending strength of the pole at the location of the access port.
Because of this tendency of the utility poles to buckle in proximity to the location of the access ports when subjected to compressive stresses, standards which delineate the magnitude of the bending stresses which utility poles must withstand have been imposed upon utility poles used in conjunction with federally funded highways. In general, such poles must be capable of withstanding two times the maximum compressive loading imposed on the pole by the maximum anticipated wind load calculated in conformity with ANSI C-136.2 standards. The specific testing routine is also delineated by that accepted governmental standard.
The specific tests have, heretofore, been difficult to satisfy without adding undue wall thickness, and the concomitant additional weight, to FRP utility poles.